The main headwaters of Belmont Creek are located in the western hills, west of Belmont. There are two other tributary creeks that feed Belmont Creek: The Alameda Canyon Creek that currently flow behind Carlmont Shopping Center and the north fork of the Belmont Creek that flows out of the northern hills of Belmont, across the Ralston Estate merging just west of the current Twin Pines Park. Belmont Creek is fed by drainage of rainwater from the western hills, the tributaries and some natural springs in the area. Belmont Creek flows from the western hills to the bay lands, flowing east down Ralston canyon (In the early years Canada del Diablo) to the bay.
Belmont’s open space plays a major role in controlling storm water runoff and preventing flooding. Five square miles of watershed drain through the City of Belmont and Belmont Creek is the largest drainage, carrying about 62 percent of the flow through the city. Its headwaters are in the hills above Hallmark Drive and it runs roughly parallel to Ralston Avenue through Water Dog Lake and Twin Pines Park. The creek exits the City just above Old County Road at Harbor Boulevard and then forms the boundary between the Harbor Industrial Area and the City of San Carlos. It reenters Belmont in the Island Park neighborhood as Belmont and O’Neill Slough before discharging to San Francisco Bay. Over time, development has greatly increased the amount of storm water entering the creek. Without the open space to soak up large amounts of rain water, the creek would quickly become overwhelmed and flooding potential would greatly increase.